A hybrid vehicle has an internal combustion engine and an electric motor as motive power sources for driving. For example, such a hybrid vehicle travels selectively in an engine-driven mode, a motor-assisted mode, or a charging mode. In the engine-driven mode, only the engine power drives the vehicle. In the motor-assisted mode, both the engine power and the motor power drive the vehicle. In the charging mode, the vehicle travels while its battery is charged by the electric power generated by the motor.
For example, Japanese Patent No. 3662904 discloses a system for switching the foregoing modes in accordance with the residual power, that is, state of charge (SOC), of the battery of such a hybrid vehicle so that the driving force necessary for driving the vehicle can be shared suitably between the engine and motor of the vehicle. In the motor-assisted mode, this system selects the motor driving force ratio, which is highest in the efficiency of fuel consumption, with respect to electric power consumption. In the charging mode, this technique selects the motor driving force ratio highest in the efficiency of fuel consumption with respect to electric power generation. This system determines the motor-assisted mode, the charging mode or another mode in accordance with the residual power of the battery by means of a map. This system also determines the motor driving force as a parameter with which the fuel consumption in the engine can be reduced most.
However, the energy efficiency during motor drive also varies with the amount of fuel consumed when the battery is charged by the electric power generated by the motor. This reduces the accuracy in energy efficiency calculation, because the amount of fuel consumed at the time of electric power generation is not taken into consideration. The reduction in accuracy limits the driving force from being shared properly between the engine and the motor. As a result, it is impossible to reduce the fuel consumption optimally.